Determination of the rate of the glutamate/glutamine cycle in the human brain by in vivo 13C NMR

Abstract

Recent 13C NMR studies in rat models have shown that the glutamate/glutamine cycle is highly active in the cerebral cortex and is coupled to incremental glucose oxidation in an approximately 1:1 stoichiometry. To determine whether a high level of glutamatergic activity is present in human cortex, the rates of the tricarboxylic acid cycle, glutamine synthesis, and the glutamate/glutamine cycle were determined in the human occipital/parietal lobe at rest. During an infusion of [1-13C]-glucose, in vivo 13C NMR spectra were obtained of the time courses of label incorporation into [4-13C]-glutamate and [4-13C]-glutamine. Using a metabolic model we have validated in the rat, we calculated a total tricarboxylic acid cycle rate of 0.77 +/- 0.07 micromol/min/g (mean +/- SD, n = 6), a glucose oxidation rate of 0.39 +/- 0.04 micromol/min/g, and a glutamate/glutamine cycle rate of 0.32 +/- 0.05 micromol/min/g (mean +/- SD, n = 6). In agreement with studies in rat cerebral cortex, the glutamate/glutamine cycle is a major metabolic flux in the resting human brain with a rate approximately 80% of glucose oxidation.

Figure 1

Schematic representation of a two-compartment modeling of glutamate/glutamine neurotransmitter cycling between astrocytes and neurons. CMR_glc, cerebral metabolic rate of glucose; V_ana, anaplerosis flux, gV_tca, glial tricarboxylic acid cycle flux, V_cyc, glutamate/glutamine cycling flux, nV_tca, neuronal tricarboxylic acid cycle flux.

Figure 2

In vivo ¹³C spectrum from the occipital/parietal lobes of a healthy volunteer using ¹H-localized adiabatic polarization transfer technique at 2.1 Tesla. The spectrum was an accumulation of 67.5 min of acquisition 60 min after the start of 1-¹³C-glucose infusion. Homonuclear peaks attributable to [3-, 4-¹³C]-glutamate were clearly resolved from the central [4-¹³C] glutamate and [3-¹³C] glutamate peaks. Labeled resonances are [4-¹³C]-glutamate (Glu4) and [4-¹³C]-glutamine (Gln4), [3-¹³C]-glutamate (Glu3), and [3-¹³C]-glutamine (Gln3), respectively. Other resonances present in the spectrum include [3-¹³C]-lactate at 21 ppm, the sum of the resonance of [2-¹³C]-GABA and the downfield resonance of the ¹³C-¹³C satellite of [4-¹³C]-glutamate at 35 ppm, the sum of the resonance of [4-¹³C]-GABA and N-acetyl aspartate at 41 ppm, and the resonance of [3-¹³C]-aspartate at 37 ppm.

Figure 3

A time course of the concentrations of [4-¹³C]-glutamate and [4-¹³C]-glutamine for a single subject. The fit of the two-compartment model shown in Fig. 1 to the data also is shown. Asterisks, glutamate; open circles, glutamine.

Figure 4

The time course of the summed data of all six subjects for the fractional enrichments of [4-¹³C]-glutamate and [4-¹³C]-glutamine and the best fit of the two-compartment model. The enrichment of [4-¹³C]-glutamine is clearly seen to lag the enrichment of [4-¹³C]-glutamate, consistent with neuronal glutamate being the main precursor for glutamine synthesis via the glutamate/glutamine cycle. All fractional enrichments were normalized to that of [4-¹³C] glutamate. Asterisks, glutamate; open circles, glutamine.